1. Field of the Invention
The present invention relates to a method for manufacturing a sintered body with high density from a ceramic porous body, and more particularly to a method of forming a layer on the surface of the porous body.
2. Description of the Prior Arts
The HIP process (hot isostatic pressing sintering screen) is well known wherein, firstly, metallic or ceramic powders are formed into a compact of a predetermined shape, and the compact is presintered into a presintered body then, a gaseous medium at high temperature and high pressure, such as argon and nitrogen, is applied to the compact or the presintered body whereby the compact or the presintered body is sintered into a sintered body with high density. In this HIP process, such a porous body as the compact or the presintered body is put into a pressure vessel to which high temperature and high pressure are applied and is isostatically pressed by means of a gaseous medium at 600.degree. C. to 2,500.degree. C. and 500 to 3,000 atm. Thus, the porous body is increased in density.
However, owing to this HIP method using a high pressure gas as a medium for applying pressure, it is necessary to prevent the gas from penetrating into the pores of the porous body. As such a measure, the following are known:
(a) a method wherein density of a presintered body is increased to 93% or more of its true density, and more preferably to 95% or more and a presintered body itself is transformed into one impermeable against gas; and
(b) a method wherein a compact or a presintered body is arranged in a sealed capsule impermeable against gas, and a gas medium at high temperature and high pressure is applied thereto, whereby the density is increased. This method is applicable to an ordinary compact which has a density of 40 to 75% of true density and is inevitably permeable, or a presintered body which has a density less than 93% of true density.
In method (b), sometimes a porous body is arranged in a capsule prepared in advance and then the capsule is sealed. But, this way is hard to apply to a porous body having a complicated shape and the application of this method is limited to porous body of a simple shape.
In addition, sometimes in method, a powder layers are formed on the surface of the porous body, and heated and softened to have the powder layer transformed into a tightly sealed capsule. When this method is employed, the restriction arising from the shape of the porous body is small, and is particularly useful, for a porous body having a complicated shape.
In Japanese Examined Patent Publication (KOKOKU) No. 35870/84 there is dissolved a process wherein:
(a) on the surface of a preformed body of silicon nitride powders an inner porous layer of a first material and an outer porous layer of a second material are formed;
(b) heat treatment is applied whereby the temperature is high enough to allow the outer porous layer to be transformed into one impermeable against a pressure medium, but at which the inner porous layer still maintains its porous property;
(c) heat treatment is further applied up to a temperature high enough to transform the inner porous layer into one impermeable against a pressure medium;
(d) to the preformed body having had these heat treatments through the inner porous layer and the outer porous layer, isostatic pressing is applied; and
(e) the first material constituting the inner porous layer is a powder of high-melting glass, a powder capable of forming a high-melting glass, or a powder of high-melting metal, and the second material constituting the outer porous layer is a powder of low-melting glass, or a powder capable of forming low-melting glass.
There is also disclosed in Japanese Patent Unexamined Publication (KOKAI) No. 116178/84 a process wherein a ceramic porous body is covered with seal material composed of porous high silicate glass or nitride thereof and is sintered at high temperature and at high isostatic pressure to produce a sintered ceramic body with high density.
Further, another prior art method is disclosed in West German Pat. No. 3403917. In this specification, powder layers are formed, on the surface of a porous body, composed of two layers, i.e. an inner layer and an outer layer. The inner layer is of substance not containing a sintering-promoting additive, and the outer layer is of substance containing a sintering-promoting additive. Consequently, the outer layer has characteristic for transforming into an impermeable layer. The inner layer, as a separated layer hard to sinter, has characteristic of making it easy to remove the impermeable layer which has been formed.
These methods, however, have a problem in reliability in formation of a layer, since a preformed porous body is coated with slurry which is made by means of dispersing powders into a solvent. That is to say, since powders are usually cohered if they are used simply an directly in producing slurry, big pores of 30 to 40 microns are brought about in the formed powder layers. Owing to these pores remaining in impermeable layers, formation of the impermeable layers is impaired. Therefore, dispersing the powders is indispensable. In addition, if selection of a solvent is not appropriate, owing to the powders precipitating, formation of layers having uniform density and thickness is impaired. Furthermore, shrinkage occurs during the drying process after the coating and cracking can easily occur. If, in order to lessen the shrinkage, the powder concentration in the slurry is increased, adhesion to the porous body is insufficient. As a result, the layers easily separate to but also lack uniformity in thickness. To avoid the shrinkage and cracking, addition of organic binder is effective, but defects are apt to occur when the organic binder is pyrolyzed. Owing to these various reasons, it has been difficult to form layers reliable enough to maintain impermeability against high pressure.